1. Field of the Invention
The present invention relates to a method and apparatus for controlling power supplied to a laser diode to an optimal level in an optical recording/playback apparatus, and more particularly, to a method and apparatus for controlling the power supplied to the laser diode, so that fluctuation in the power of the laser diode, which occurs at a record/playback area of an optical disc, can be prevented.
2. Description of the Related Art
With the advent of multimedia, the demand for high-density recording media has increased. Such high-capacity recording media include magnetic optical disc drives (MODD), digital versatile disc-read only memory (DVD-ROM) drives, DVD-random access memory (DVD-RAM) drives and the like.
Such optical recording apparatuses must be optimized with high-accuracy depending on the type of optical recording medium used. In order to ensure accurate recording/playback of data, such optical recording apparatuses include an automatic laser diode power control (APC) circuit for controlling the power applied to a laser diode (LD).
FIG. 1 is a block diagram showing the structure of an APC circuit in a conventional optical recording apparatus. In FIG. 1, reference numeral 120 represents a write waveform generator for generating write pulses to form spaces and marks according to data to be recorded, reference numeral 140 represents an LD driver for controlling the power of an LD 160 according to the write pulses generated by the write waveform generator 120. Data to be recorded is input to the write waveform generator 120, in the form of a non return to zero inverted (NRZI) signal. Also, in the case of a DVD-RAM, a combination of a first pulse, a last pulse, a cooling pulse and a multi-pulse train is output.
An APC circuit 200 includes a photo diode (PD) 202, a variable gain amplifier (VGA) 204, a comparator (COMP) 206, an up/down counter 208 and a digital-to-analog converter (DAC) 210. The PD 202 receives laser beams reflected by a disc 180, and generates a current signal corresponding to the amount of the received laser beam. The VGA 204 amplifies the current signal from the PD 202 to a predetermined gain and converts the current signal into a voltage signal. The COMP 206 compares the output from the VGA 204 to a reference voltage Vref, and outputs a binary decision signal indicating which one is higher than the other, according to the comparison result.
The up/down counter 208 performs up or down counting based on the comparison result from the COMP 206. If the decision signal from the COMP 206 represents that the output from the VGA 204 is higher than the reference voltage Vref, for example, if the decision signal is low, the up/down counter 208 performs up-counting. Meanwhile, if the decision signal represents that the output from the VGA 204 is lower than the reference voltage Vref, for example, if the decision signal is high, the up/down counter 208 performs down-counting. The count result from the up/down counter 208 is applied through the DAC 210 to the LD driver 140. The LD driver 140 controls the amount of power supplied to the LD 160 according to the amplitude of the signal applied from the DAC 210.
The APC circuit 200 of FIG. 1 detects the difference between the output level of the LD 160, which is detected by the PD 202, and the reference voltage, and controls the power supplied to LD 160 in accordance with the detected difference. However, in the APC circuit 200 of FIG. 1, the output from the LD 160 continuously changes even at an effective data area of the disc 180 due to a continuous operating characteristic of the up/down counter 208. Thus, it is difficult to obtain an accurate recording/playback result. In addition, the APC 200 is susceptible to external noise.